海角大神

All eyes are on Comet Borisov. But this isn鈥檛 just any comet.

The cosmic block of ice, rock, and gas hurtling toward our sun came from another solar system. It鈥檚 only the second of its kind ever spotted.听

The first interstellar visitor, a peculiar object dubbed 鈥極umuamua, flashed through the night sky in October 2017. Scientists didn鈥檛 expect to see another until after 2022. But Borisov is here now, burning bright in a cloud of dust and cyanogen gas that makes it closely resemble our solar system鈥檚 comets.听

Together, these visitors herald a new frontier in astronomy 鈥� one where the universe is coming to us, rock by rock, providing clues about our neighbors in the Milky Way. The presence of these interstellar travelers in our solar system also adds a new dimension to our understanding of the cosmos.听

鈥淚t shows right away that these objects are probably a lot more common than people had thought they were,鈥� says Robert Weryk, an astronomer at the University of Hawaii at Manoa, who discovered 鈥極umuamua. 鈥淎nd now that we know they鈥檙e there, there鈥檚 going to be a much more active search to find them and study them.鈥�

What happened when we stopped looking

These interstellar visitors are generating a lot of buzz among 21st-century scientists. But if you went back about 400 years, 鈥渘o one would have batted an eye,鈥� says Sara Schechner, a historian of astronomy at Harvard University.听

Philosophers at the time conceptualized comets and asteroids as objects that hurtled around the galaxy, not necessarily bounded by fixed star systems. But as modern notions of a vast and orderly universe emerged, interstellar traffic fell out of fashion.

In the 20th century, a new barrier for where a comet could go was introduced: the Oort cloud, a theoretical shell of icy objects that surrounds our solar system. A collision within the cloud could send an object flying deeper into the solar system, but it鈥檚 much harder to push an object outward, against the sun鈥檚 gravity. If the same structure exists around other star systems, scientists surmised, it would be very difficult for a comet to escape a star system, let alone travel the unfathomable distances and survive a trip through our Oort cloud.听

In short, says Dr. Schechner, interstellar comets became so unlikely, we stopped looking.

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Scientists have probably seen objects like Borisov or 鈥極umuamua before but dismissed them as observational errors, says Javier Licandro, a planetary scientist at the Instituto de Astrof铆sica de Canarias in the Canary Islands.听

But听Borisov听was no mistake.听

A new window into the cosmos听

An amateur astronomer in Crimea first spotted Borisov on Aug. 30. Other astronomers around the world quickly began tracking its trajectory, eager to determine if it was a second interstellar visitor.听After several days, the International Astronomical Union confirmed that the comet did indeed come from another star system.

While 鈥極umuamua hung around for about a week, Borisov is projected to be observable well into 2020. Its听path is projected听to take it closest to our sun on Dec. 8, though comets can behave unpredictably in heat as gases inside it burn, creating high-pressure jet streams that can push the comet off course. The sun鈥檚 heat could also disintegrate Borisov altogether.听

Whatever happens, scientists are tracking the comet closely. Dr. Licandro is particularly focused on capturing Borisov鈥檚 journey. His lab was one of the first to听capture high-resolution听images of our guest, and they will continue to observe as it exits our neighborhood. 鈥淭his is a unique opportunity to see how an object forms around another star,鈥� he says.听

The first two interstellar visitors present many puzzles: Where do they come from? How frequently do such objects fly through our solar system? And, how many have we missed?听

The differences between Borisov and 鈥極umuamua make things even more interesting. While Borisov has an aura and tail familiar to comet scientists, 鈥極umuamua appeared as a dim, oblong rock that tumbled end over end. 鈥淏eing able to contrast the two really tells you something about these objects,鈥� Dr. Weryk says 鈥� and these objects are one of the few ways we can connect with the world beyond our solar system.听

Some scientists suggest the interstellar objects 鈥� and this is completely serious 鈥� might be alien sent. Even without aliens aboard or sending them as envoys, space rocks flying between solar systems offer another possibility for exchange: Could comets carry cosmic material from one side of the galaxy to another, like dandelion seeds? Scientists have pondered whether these interstellar visitors might influence the development of solar systems, define the chemistry of young planets, or even seed life across the universe.听

Fascinated by these mysteries, and inspired by Borisov鈥檚 early detection, many scientists are already听thinking about future guests. One tool in development that could help comb the starscape for interstellar objects is the听Large Synoptic Survey Telescope听in Chile, which, if all goes according to plan, will take nightly听wide-angle snapshots of the sky starting in the next few years. Other organizations, like the European Space Agency, are working on听comet interceptors that may one day visit an interstellar guest.听

For Dr. Weryk, future visitors are definitely on the mind. 鈥極umuamua caught him by surprise when he discovered it in 2017. But now, every time he spots a new space rock, he wonders: Is this from interstellar space, too?听

鈥淭hat鈥檚 something I never would have considered before we knew they existed for sure,鈥� he says. 鈥淧retty much every candidate we get now, it鈥檚 like, oh, maybe this could be the next one.鈥�